Simultaneous near-field optical birefringence and fluorescence contrast applied to the study of dye-doped polymer-dispersed liquid crystals

The local optical and electrooptical properties of dye-doped polymer-dispersed Liquid crystal (PDLC) thin films are explored by near-field scanning optical microscopy (NSOM). Detailed information on the mechanisms for dye alignment and electric-field-induced- dye reorientation in localized sample regions is sought. Dye reorientation is predicted to occur by two distinct processes: (i) via its interactions with the Liquid crystal host and (ii) via the direct interactions of its permanent and induced dipole moments with the,applied field. The liquid crystal is doped with a nearly isotropically aligned BODIPY dye so that contributions of both reorientation mechanisms can be explored. An order parameter of 0.10 +/- 0.02 is measured for BODIPY in bulk aligned samples, suggesting that host-guest interactions are very weak. Simultaneously recorded birefringence (transmission) and fluorescence NSOM images confirm that the dye is apparently not well aligned, even on submicrometer length scales. Surprisingly, NSOM studies of dye reorientation prove that host-guest interactions still play a substantial role in controlling the dye reorientation process. The field dependence of dye reorientation is found to be correlated with liquid crystal reorientation, varying spatially across individual droplets. Analysis of the simultaneously recorded birefringence and fluorescence data shows that the dye is strongly reoriented via its interactions with the reorienting liquid-crystal host and also via its independent interactions with the applied field. The former mechanism is manifested as a rotation of the dye alignment axis while the latter leads to a field-dependent increase in the local dye order parameter. "Bistable" alignment of the dye in the nematic host is proposed as a possible explanation for the apparent coupling of the two reorientation mechanisms.